The present invention relates to a load drive circuit for driving a load circuit, which is controlled to be turned ON/OFF by a switch operated by a user.
A lamp of a direct off switch system directly controlled to be turned ON/OFF by the operation of the user is loaded on an automobile or the like.
FIG. 1 shows a conventional load drive circuit, which drives a lamp load 100 by supplying a power supply voltage from a power supply 101 thereto. This load drive circuit has a constitution in which power supply management for preventing a battery from dying or the like caused by forgetting to turn off of the lamp switch 102 while a lamp switch 102 is kept ON. In the load drive circuit, when the power supply management is performed, a state of the lamp switch 102 disposed on the downstream side of the lamp load 100 seen from the power supply 101 is read by a microcomputer 111 incorporated in a control unit 110, and a relay 112 is controlled by the microcomputer 111 on the upstream side of the lamp load 100 to supply a power supply voltage from the power supply 101 to the lamp load 100.
However, in the foregoing load drive circuit, if all the circuits can be constructed on the upstream side of the lamp load 100, i.e., on a power supply voltage supply side, the construction of circuitry itself is highly efficient. However, since it is necessary to connect the lamp switch 102 with the microcomputer 111, and monitor an ON/OFF state of the lamp switch 102, overall constitutional efficiency is not so high.
In addition, since another circuit must be added to monitor the state of the lamp switch 102 by the microcomputer 111, costs are increased correspondingly.
The present invention was proposed in consideration of the above-described problems, and it is an object of the invention to provide a load drive circuit capable of driving a load circuit and monitoring an ON/OFF state of the load circuit by simple circuitry.
To achieve the foregoing object, in accordance with an aspect of the invention, a load drive circuit is provided, which is disposed on a downstream side of a power supply to supply a power supply voltage to a load circuit, and adapted to monitor a state of a load power supply switch disposed on a downstream side of the power supply of the load circuit. This load drive circuit comprises: a semiconductor switching device including a power supply terminal connected to the load circuit, and turned ON/OFF to supply a power supply voltage to the load circuit according to a load driving control signal; a monitoring switch circuit turned ON/OFF according to a monitoring control signal of a timing for monitoring the state of the load power supply switch; a monitoring resistor connected to the monitoring switch circuit; and controller for driving the load circuit by supplying the load driving control signal to the semiconductor switching device to control turning ON/OFF, and monitoring the state of the load power supply switch by supplying the monitoring control signal to the monitoring switch circuit to control turning ON/OFF, based on a voltage value by the monitoring resistor.
According to the first aspect, since the load power supply switch is operated to supply the power supply voltage to the load circuit, the semiconductor switching device is driven. Also, the monitoring switch circuit is driven to monitor the state of the load power supply switch operated to turn ON/OFF the load circuit. Thus, in the load drive circuit, supply of the power supply voltage to the load circuit is controlled by the voltage value of the monitoring resistor.
In accordance with a second aspect of the invention, the controller desirably sets supply time of the load driving control signal supplied to the semiconductor switching device to be sufficiently shorter than supply time of the monitoring control signal supplied to the monitoring switch circuit.
According to the second aspect, a period of monitoring the load power supply switch by the monitoring control signal is set to be shorter than a period of driving the load circuit by the load driving control signal.
In accordance with a third aspect of the invention, the controller desirably turns OFF the semiconductor switching device during driving of the load circuit, and turns ON said monitoring switch circuit to monitor the state of the load power supply switch while turning OFF the semiconductor switching device.
According to the third aspect, while the load power supply switch is ON to drive the load circuit, the semiconductor switching device is first turned OFF, and the state of the load power supply switch is monitored within this period.
In accordance with a fourth aspect of the invention, the load circuit may be a lamp load.
According to the fourth aspect, the semiconductor switching device is driven to supply a power supply voltage to the lamp load, and the monitoring switch circuit is driven to monitor the state of the load power supply switch operated to turn ON/OFF the lamp load.